Method and apparatus for alternating-current investigation of uncased drill holes



April 21, 1936. J J, JAKOSKY 2,038,046

' METHOD AND APPARATUS FOR ALTERNATING CURRENT INVESTIGATION OF UNGASED'DRILL HOLES Filed Jan. 12, 1954 5 Sheets-Sheet l April 21, 1936.

J. J. JAKOSKY 2,038,046 METHOD AND APPARATUS FOR ALTERNATING CURRENTINVESTIGATION OF UNCASED DRILL HOLES Filed Jan. 12, 1954 3 Sheets-Sheet2 April 21, 1936.

- 5 SheetsSh eet 3 J. J. JAKOSKY Filed Jan. 12, 1954 INVESTIGATION 'OFUNCASED DRILL HOLES METHOD AND APPARATUS FOR ALTERNATING CURRENTPatented Apr. 21, 1936 UNITED STATES PATENT Q The object of theinvention is to improve the bearing sands. procedure now employed and toincrease the ac- The second and third methods utilize highcuracy of thedeductions which may be made frequency radiation, but the radiation fromsuch when interpreting the results of the investigation. systems isgoverned by the mass effect of the Previous methods have been proposedfor sub-.- subsurface, and. no means are provided for dif- 10 surfaceinvestigations in bore-holes. The most ferentiation or measurement ofthe various strata. prominent of these methods involves measure-Furthermore, in the case of the third method, ment of the specificresistivity of the various the radiation resistance is governed toalarge strata through which the drill-hole has peneextent by theposition of the radiating system trated (U. S. Patent No. 1,819,923). Inanother within the drift or tunnel and the nearness to 15 method, awire, constituting the counterpoise of the wall rocks. For obtainingreliable data re-'. a high-frequency antenna or radio system is gardingthe characteristics of the strata it is placed inside the drill hole andthen by noting essential that the radiating system be in contact thedifierencein the resistance characteristic of with the water or drillingfluid in 'the drill-hole,

the antenna at different wave lengths, predicin order to minimizesufficiently the contact re- 20 tions are made relative to the proximityof the sistance and variations due to position, and that greaterconducting bodies, as compared to similar the electrodes be. ofsufficient area or extended tests in virgin areas (U. S. Patent No.1,652,227). surface to make the potential drop at the elec- A similarsystem is proposed by U. S. Patent No. trode surfaces of smallmagnitude.

26 1,092,065 by placing a previously calibrated an- In order to overcomethe disadvantages of the 15 tenna system in parts of a mine, and notingthe above methods, I have developed a high-frecapaciiy required to tunethe apparatus to a prequency, alternating-current method foraccudetermined frequency again, or the change in the rately measuringthe impedance losses, power frequency and the damping coefficient of theapfactor, and the dielectric constants of the vari- 30 paratus. A fourtharrangement utilizes changes ous strata penetrated by the drill-hole.The 30 in potential between two surface points, as one method consistsessentially of lowering into the electrode of the current supply islowered into bore-hole a cable carrying at its lower end an thebore-hole, while the other terminal of the extended electrode system.These electrodes are current supply is placed at the surface of thesupported by an insulator spacer which retains ground (U. S. Patent No.1,863,542). them in rigid spatial relation to each other. They 35 Thefirst and fourth methods (resistivity) are also connected, by insulatedconductive wires above described depend for their operation (whenpassing through the cable from which they are used in oil fields)chiefly upon the high resistsuspended, with a source of alternatingcurrent ance of an oil-impregnated sand as compared to at the surface ofthe ground. and proper measura sand impregnated with water. By plottinga ing apparatus. Inits simplest form, the elec- 40 curve showingresistance versus depth, attempts trode systemmay comprise two extendedelecare made to determine the location and thicktrodes. The dielectricand conductive material, ness of the oil-bearing sands, by means of thewithin the sphere of the electrodes (of which a changes in resistivityvalues. Interpretation of small part is the insulative spacer or supportthese data is not always reliable since many which holds the electrodesin rigid spatial relahighly resistant strata are encountered in the tionand the fluid in the dri1l -h01e),'c0nsist prinsubsurface. This isparticularly true when very cipally of the earth materials surroundingthe dense beds such as gypsum, certain limestones, uncased bore-hole andextending laterally in all' etc, are encountered. Due to a low rate ofpenedirections to a considerable distance.

0 tration of the drilling fluid into the wall rock, The various featuresof novelty which char- METHOD AND APPARATUS FOR ALTERNAT- ING-CURRENTINVESTIGATION CASED DRILL HOLES John Jay Jakosky, Los Angeles, Calif.Application January'12, 1934, Serial No. 706,391

OF UN- 15' Claims. (01. -182) 'This invention relates to a methodandappa ratus for investigating the nature of subsurface strata byelectrical means, and in particular for conducting investigations insideof bore or 'drillholes or other uniform openings in the'earth.

the electrical resistance of such dense materials is very high andoftentimes approaches that of the oil sands. Furthermore, many oil sandscontain a certain amount of water, especially in fields where the oilhas been partially depleted and water is coming into the field. Undersuch conditions the present resistivity methods give erroneous orambiguous results and have often- .times been found very unreliable fordetermining the accurate location and thickness of oilacterize myinvention are pointed out in the appended claims. For a more completeunderstanding of the invention, however, reference should be made to theaccompanying drawings and descriptive matter. Of the drawings:

Far-issues Figure 1 is a vertical section of the earth showing a portionof a drill-hole and one form of electrode system and supporting cable.

Figure 1A is a diagrammatic representation of apparatus which maybe usedin measuring the impedance losses of the subsurface strata.v

Figure 1B is a diagrammatic representation of apparatus useful formeasuring the impedance and the dielectric value of the subsurfacestrata.

Figure 1C is a diagrammatic representation of apparatus which may beused in measuring the power factor anomalies caused by the subsurfacestrata.

Figure 2 illustrates a simple electrode system which may be employedwhen using a cable con sisting of only one insulated conductor and anouter steel supporting sleeve or braid.

Figure 3 illustrates a form of recording apparatus for obtaining acontinuous record of variations in the subsurface as the electrodesystem traverses the drill-hole.

Figure 4 is a diagrammatic representation of apparatus useful forenergizing the earth by two subsurface power electrodes, and measuringthe potentials between two auxiliary electrodes.

Figure 5 is a diagrammatic representation of another form of apparatususeful in measuring impedance and capacity variations.

Figure 6 is a diagrammatic representation of apparatus and electrodesystem for measuring variations in power factor or phase shift betweenthe current and the potential circuits, when energizing the subsurfaceby means of alternating current flowing between two electrodes, andusing the potential created between two auxiliary electrodes, one orboth of the energizing electrodes being separate from the two potentialelectrodes.

The high-frequency circuit, of which the electrode system and the cableof Figure 1 form a part, contains resistance, inductance and capacity,so as to satisfy the relation (assuming lumped values of L and C)L=inductance in henries C=capacity in farads.

At resonance the power factor of the circuit becomes unity and a maximumcurrent passes. The adjustment to' resonance is sharply sensitive andaffords a delicate indicator of deviations from the above relationbetween inductance and capacity.

In the forms of apparatus described, the inductance is of low value andapproximately constant, whereas the capacitance is high and appreciablyafiected by the formations surrounding the drill-hole. It is thereforeonly necessary to adjust capacitance in the various types of bridgecircuit in order to secure resonance.

In a high-frequency radiating circuit, electric energy is dissipated inthree different ways: first, energy is dissipated in ohmic resistance ofthe circuit, the loss being equal to the product of the current squaredby the resistance. Second, the circuit loses energy by radiation, thisenergy being carried by the electric wave traveling outward into space.This radiated energy will perform work by the setting-up of currents inconducting materials or circuits placed in the path of the wave. Theamount of energy radiated is proportional to the square of thefrequency. It is also proportional to the square of the current in theradiating dielectric due to the so-called dielectric hysteresis. Whenusing a sufliciently high frequency and because the surroundingdielectric is not perfect, the alternating fields extending from theradiating circuit lose energy due to its absorptioniby the medium. Thisis also known as dielectric absorption and this loss.is inverselyproportional to the frequency.

The various energy losses in the circuit may be treated mathematicallyby assuming them to be 1 R losses in real resistance or in varioushypothetical equivalent resistances.

The dielectric absorption in a given condenser system is foundexperimentally to be proportional to the energy stored in the condenserduring each half cycle. The series resistance equivalent to this lossmay be determined as follows. (Moullin, Radio Frequency Measurements, p.140, Chas. Griffin 8: (30., Ltd, London.)

R=equivalent resistance of condenser in ohms Under the conditionsencountered in use of a drill-hole survey apparatus, moreover, theinsulation of the condenser (existing between the terminal plates) isimperfect and the apparent equivalent series resistance varies inverselyas the square of the frequency as shown by the following equations for acondenser, having a non-inductive resistance 1" in parallel with it.,(Moullin, l. c.)

where Z is the impedance. The first term in the right-hand member of theequation may be interpreted as a series resistance R in phase with thecurrent flowing through the condenser, so that The condenser losses ofenergy due both to the dielectric absorption and to, the leakage, orcurrent conduction, may therefore be grouped together as a resistance inseries with the condenser, and if the power factor (cos 0) of thecondenser is small, the equivalent series resistance is givenapproximately by the equation cos 0 21rfC It will also be seen that thepower factor of the system varies with the resistance and capacity. Thehigher the frequency, the less become the effects of current conductionthrough a leaky dielectric. In order to obtain the greatest benefits ofdielectric phenomena, the preferred condition circuit. Third, energy islost to the surrounding contemplated by this invention, it is essentialthat high frequencies be employed. Under the conditionsgenerallyencountered in practice, the frequency should be from 2000cycles to as much as 1,000,000 cycles per second. Frequencies in theneighborhood of 5,000 to 20,000 cycles per second have been foundparticularly eifective. The frequency must be sufliciently high to'causea measurable shift in phase between the current and potentialcomponents.

The use of a high-frequency current also minimizes the contactresistance occurring at the electrodes. The contact resistance isusually a serious variable when measurements are made with directcurrent. This contact resistance effect is minimized, when highfrequencies are employed, probably by two factors: (1) elimination ofpolarization and electrolysis effects; and. (2) the high capacityexisting at the contact of the electrolyte (impure water of thedrill-hole) and the electrode. This electrolytic capacity effect isenhanced when the electrodes are constructed from aluminum or similaroxidizable material. In oil wells an electrolyte consisting of water anddrilling mud is always present.

The dielectric constants of earth strata vary in significant degree. Forexample a majority of the following figures were taken fromInternational Critical Tables, vol. VI, p. 105.

Granite 3 to 4 Dry sand 2.5 Wet sand H2O.) ca 9 Petroleum ca 2.5 to 3Oil-impregnated standstone 3 .to 4 Sandstone 9 to 11 Dry soil 1.9 Soil(8% H2O) ca 8' Water 80 Limestone 8 Fundamental electrical theory showsthat the capacity of a two-plate condenser is represented by theapproximate equation:

where C=capacity in cm.

K=specific inductive capacity of dielectric A=area of one side of oneplate 1 cm d=separation of plates in cm.

' (Morecroft, Principles of Radio Communication, p. 214, Wiley and Sons),Capacities of less simple forms of condenser are not readily expressedin mathematical formulae, but in general the principle holds that thecapacities are approximately'directly proportional to the specificinductive, or dielectric, capacities of their dielectrics: The aboveequation assumes a perfect dielectric, that is, one which will pass nocurrent when a steady electromotive force is applied to its terminals,and which will consume no energy when subjected to alternatingelectromotive forces. Under these theoretically ideal conditions, thepower factor is 90v degrees leading. As previously discussed,

,none of these conditions holds for dielectric earth materials. Director indirect measurement of the dielectric losses occurring inthe variousstrata through which the drill-hole penetrates therefore furnishes areliable and particularly advantageous method of logging the thicknessof the various strata, and in .particular of differentiating betweenessentially oil-impregnated materials and water-impregnated materials;and between oil-impregnated and certain impervious rocks.

In the simple application of theapparatus described later, the totalimpedance governs the effects measured. Under these conditions, no

attempt is made to differentiate between the effects of resistance,dielectric value and dielectric losses. All of these factors govern theradiation loss and the impedance. Since the relative resistances ofvarious strata vary from 6 to 15, it is impossible to obtain an idea ofthe true dielectric properties of thematerials, merely by measuring thechanges in radiation resistance of a high-frequency system, although, aspreviously mentioned, the higher the frequency, the less becomes theeffect of the resistance losses.

In the preferred method herein described, the undesired resistancecomponent may be eliminated by use of (a) a proper bridge circuit formeasuring capacity; and (b) by making measurements at differentfrequencies. In practice it has been found best to log depth-dielectricor depth-capacity measurements at one frequency during the descent intothe drill-hole, and then log a second series of measurements at adifferent frequency during the ascent. This gives two curves from whichthe dielectric property in the subsurface may be evaluated moreaccurately. The process of obtaining the depth-dielectric log may betermed dielectric coring.

Employing an approximately tuned circuit increases the sensitivity andfacilitates measurement of. the impedance, or any of its components. Theresistance ,and capacity components are the two major variable quatitieswhich change with the strata through which the measuring device passes.impedance variations gcomposed of the resistance and the capacitycomponents in space quadrature) will give data indicative of the strata,but as a general rule I prefer to carry out the field measurements insuch a manner as to determine the capacity component most accurately.This allows determination of the dielectric value of the strata, withthe resultant advantage of more accurate diiferentiation of the variousstrata. At certain' points where In some cases the more delailed datamay be desirable, it is oftentimes of advantage to carryout a complete Vseries of measurements to determine the changes in the alternatingcurrent characteristics of those particular formations at differentfrequencies. This can be done by stopping the electrode system at thedesired depth and obtaining data showing variations of the alternatingcurrent characteristics such as impedance, power factor, capacity, at'the different frequencies.

Various types of measuring device and circuit arrangement may beemployed for measuring or continuously recording the changes inalternating-current impedance, power'factor, or capacity. as theelectrodes traverse the drill-hole. These devices are well known and thefollowing descriptions of circuit arrangements are included hereinchiefly for the purpose of clarifying the field operation of the method,and should not be considered as limiting this method for use with anycertain type of measuring circuit. The sensitivity of the indicatingdevices may usually be increased if desired by the use of one or morestagesof vacuum-tube amplification.

One form of electrode system is illustrated in Figure 1 and comprisesmetal electrodes l and 2 spaced a predetermined distance apart on aninsulator tube 3 and 3'. The upper end of insulator 3 is mechanicallyfastened to the steel braid forming the outer sleeve 4 of an insulatedtwo-conductor cable. The sleeve 4 is of sufficient mechanical strengthto support the entire weight of the cable and allow the proper factor ofsafety. Concentric insulated conductors 5 and 6 of the cable areconnected to the terminals I and 2. The cable may be fastened'to asuitable reel or drum (not shown) to allow the assembly to be lowered orraised within the drillhole. The inside ends of the cable on the drum,constituting the other ends of conductors 5 and 6, may be connected to acommutator fastened to the drum, and thence to an electrostaticwattmeter and alternating-current power supply for measuring theimpedance changes of the system or to an alternating' current bridge formeasuring the capacity changes of the system, or to a power-factor meterfor measuring the changes in power factor of the system. The outer steelsleeve 4 of the cable is grounded.

The electrostatic wattmeter, power-factor me,- ters and the bridges areof the conventional type and need not be described in complete detailhere. Brief outline of the circuits, however, are given later. Forfurther .particulars see Alternating Current Bridge Methods by B. Hague,Pittman and Sons, Ltd, and absolute Measurements of Capacity, Bureau ofStandards, 1904, vol. 1, and Dictionary of Applied Physics by R.Glazebrook, vol. 2, Electricity.

For measuring the changes in impedance as the electrode system traversesthe drill-hole the apparatus illustrated in Figures 1 and 1A isemployed. In Figure 1A is shown a high-frequency, alternating-currentpower supply 50, voltage dividers 5| and 52, electrostatic wattmeter 53,and shunt resistor 54. The electrostatic wattmeter is preferably of thecontinuous photographic recording type, whereby a complete record ismade of the variations in impedance as the electrode system traversesthe drillhole.

For measuring the changes in dielectric values, the apparatus indicatedin Figures 1 and 1B are preferred. Referring to Figure 13: two

branches are similar resistance members land 8, the other two branchesof the bridge are composed of a calibrated variable condenser 9 and acalibrated variable resistor I0. The inherent capacity, inductance andresistance of the cable itself and of the electrode assembly with thesurrounding strata constitute the remaining branch of the conventionalbridge. These components are represented diagrammatically in thedrawings by the dotted lines indicating capacity I I, inductance I2 andresistance I3.

Alternating current is supplied the bridge terminals A and B from ahigh-frequency generator I4. This generator may be of any desired type,but for purposes of illustration I have shown a. conventional,self-excited vacuum-tubea oscillator circuit I5, coupled to the bridgeby means of a transformer I6, having variable coupling. .A calibratedwave-meter circuit comprising an inductance I1 and a calibratedcondenser I8, having loose coupling with the circuit I5, is

provided for determining the frequency of the system. A neon tubeindicator" I9 is placed in the wave-meter circuit, to indicateresonanceconditions. If desired, 'crystal control may be em-' ployedforautomatically maintaining a constant frequency in the oscillator. I

l-Any suitable type of indicating or recording instrument may be usedfor showing the balance or magnitude of unbalance in the bridge circuit.For purposes of illustration I have'shown an electrostatic voltmeter 20connected between the terminals C and D of the bridge.

For measuring the changes in power factor, due to variations in theresistance and capacity comlponents of the subsurface strata, I mayemploy the apparatus shown in Figure 10, in conjunction with the cableand electrode system shown in Figure 1. Alternating-current power issupplied by generator 50. The power-factor meter 55 is composed of twosimilar fine-wire coils 56 and 51, placed at right angles, and supportedon a pivot; suitable pointer and scale being provided. This movablesystem is placed within the field of a fixed coil 58 which carries thecurrent supplied the electrode system. In series with coil 56, is aresistor 56', and in series with coil 51 is an inductance 51'. Powerfactor is indicated by the combined efiects of the fields from the coils55, 51, and 58, and their phase relations, in accordance with well-knownphenomena.

Before use of the equipment shown in Figure. 1, it is desirable to makea preliminary calibration. This is usually accomplished in the followingmanner. The electrical characteristics of the cable and drum assemblyare determined at the various frequencies at which it is desired tomakemeasurements. These are usually obtained by short circuiting theterminals I and 2 through a resistance having a value comparable withthe resistance of the water in the. drill-hole, and lowering theassembly into 'a drill-hole. The capacity and dielectric loss of thecable system will also vary with the depth to which the cable issubmerged, due to the high mechanical pressure exerted by the drillingfluid or mud in a deep drilli hole. Simple calculations are now made andcurves plotted to obtain the normal characteris:- tics of the system.Knowing the size and shape of the electrode system, calculations can bemade to evaluate the changes in dielectric (or other) properties,encountered in surveying a well, per unit volume of material adjacentthe electrodes.

In the surveying of a well for dielectric measurements, readings areusually taken at one frequency, as for illustration, 10,000 cycles, asthe apparatus is lowered into the hole. A continuous graph is plottedshowing the relationship between the electrostatic voltmeter 20 (Figure13) reading versus the depth. Upon reaching bottom,

the energizing frequency is changed to another value, for illustration,50,000 cycles per second, and a similar set of readings obtained on theascent of the cable. The curve obtained on the descent and the curveobtained on the ascent are both preferably used for determining thedielectric value of the materials comprising the subsurface,

If desired, the impedance apparatus shown in Figure 1A and thepower-factor apparatus shown in Figure 1C may be utilized for obtainingdata regarding dielectricchanges. by making measurements at onefrequency during descent into the well, and another set of measurementsduring ascent of the well. Knowing the electrical constants of the cableand associated equipment, and

the two frequencies employed, calculations may be made to determine thedielectric properties of the strata traversed by the drill hol Analternative form of electrode assembly is shown in Figure 2 and consistsof a single insulated conductor 5' having a steel supporting andgrounding sleeve 4. The grounded sleeve of the cable forms one of theconductors. On the lower end of the cable is fastened the terminal I,and an insulator tube- 22', having a length approximately ten times thediameter of the drill hole. At the lower end of the insulator 22 isfastened an extended electrode 2', electrically connected to theinsulated conductor 5'. Measurements are made as outlined above. In thiscase the flow of the high-frequency alternating current is from theterminal 2, mainly to the lower end of the cable sleeve and the terminalfastened thereto.

Various types of indicating or recording equipment may be employed forthe surface measurements. In Figure 3 is illustrated a manually operatedcondenser, mechanically connected to a recording stylus, wherebyconstant records may be made of the variation in capacity, as theelectrode system traverses the drillhole. The cable 4 passes throughmeasuring wheels 4| and 4|; connected by means of a flexible coupling 42to a drive sprocket 43, engaged with a recording tape .44, having properperforations to receive sprocket 43. The condenser 45 is of a variabletype and manually operated by means of knob 46. Fastened to the knob orits shaft is a pinion 41, engaging rack 48. The rack'operates aconstantly recording pen or pencil 49 which describes a continuousrecord on the recording paper 44. The recording paper 44 is movedforward in accordance with movements of the cable through the measuringwheels 4| and 4|. The drive 42 may be connected to either wheel 4| or4|, depending upon direction of rotation desired. By means of thisarrangement an operator can make a continuous graph of the dielectriccapacity of the subsurface materials byproperly operating knob 46 inorder to maintain null point readings on the bridge indicating device20, shown in Figure 1B. The wattmeter 53 of Figure 1A, and thephase-meter 55 of Figure 1C, are preferably of the continuousphotographicrecording type.

An alternative system is illustrated in Figure 4, and utilizesmeasurement of the potential existing between two electrodes, whereinthe path of a high-frequency current flow. Measurements at two or morefrequencies allow the approximate dielectric constants of the variousmaterials, in the vicinity of the electrodes, to be calculated.Referring to Figure 4, a sealed tubular metallic housing 24 contains thenecessary apparatus 29, for generating the high-frequency current, andthe potential measuring apparatus 29'. This housing also forms one ofthe energizing electrodes. At the upper end of the housing is fastenedan insulative support 28, on which are placed three other extendedelectrodes. The

uppermost electrode 21, and the housing 24, comprise the two energizingelectrodes. These electrodes are electrically connected to thehigh-frequency supply. The two electrodes 26 and 25 constitute the twopotential electrodes and are the ground, for noting the variationsin'potential set up in the thermocouple. A clock mechanism 39, or otherswitching means, is provided for connecting an auxiliary capacity 34' inshunt with the initial tuning capacity 34, in order to shift thefrequency of the energizing circuit. This clock mechanism is usuallytimed to :allow measurements to be" made at one frequency during descentinto the well, and at another frequency. during ascent from the well. Itwill be seen that the potential existing across the inner electrodeswill depend not only upon the impedanceof the material through which thesystem is passing, but also upon any variations in the current flowbetween the outer electrodes. No provision is made-in this case for aconstant current system and as a result, we obtain distorted values thatare not in linear proportion to the high-frequency impedance of themedium. The values are distorted to a higher power and do not have thelinear relationship which would exist if a constant current system wereemployed. In

. be employed. In this case the source of highfrequency power 29 isconnected to the arms of a bridge circuit comprising similar resistances31 and 31. In the other leg of the bridge is a condenser 38, resistor39; and the winding of a transformer 40. The secondary winding 36 of thetransformer is connected to the two extended electrodes 24' and 35. Aninsulator support 22' is provided for properly insulating the 'terminal35 from the braided shield of the cable and its terminal clamp. Theindicating device preferably consists of an electrostatic voltmeterconnected to the bridge by means of the cable 33.

In Figure 6 is illustrated diagrammatically a system advantageous forobtaining variations in power factor as the electrode system traversesthe drill-hole. A two-conductor cable, having insulated wires 59 and 60,and a grounded steel supporting sleeve 6|, serves to connect theelectrode system with the surface apparatus.- Power is suppliedthroughelectrodes 62 and 63, connected to-conductors 59 and 6| respectively.Potential is measured between electrodes 64 and 63, connected toconductors 60 and 6| respectively. The

power-factor measuring instrument is of a modified type shown in Figure1C. The in-phase potential coil 56 is connected by the conductors 60 and6| to the electrodes 64 and 63. The reactive coil 51 is shunted acrossthe alternating current supply 50. The load current coil 58 is connectedin series with electrode 63, by means of conductor 6|. This apparatusallows data to be obtained showing the variations in power-factor orphase relations of the current flowing between the electrodes 62 and 63,and the potential existing between electrodes64 and 63. The greater thedielectric value of the materials adjacent the electrode system, thegreater will be the power-factor or phase shift between the current and'potenf .These data therefore allowv differentiation tial. of thesubsurface materials.

In practically all resistivity measuring devices, wherein lowfrequencies and direct current are.

employed, it is practically impossible to obtain a constant contactresistance. When employing high frequencies, however, I have found thata practically constant, high-frequency impedance can be obtained byusinglarge extended electrodes. This is due to the very high capacityexisting at the surface of the electrodes, which allows a high-frequencycurrent to flow without any undue contact resistance. The use of largeextended electrodes in a measuring instrument of this type is essentialto proper operation of the equipment and constitutes an importantfeature of this development. When using the two-electrode system shownin Figure l, and modifications thereof, it is preferable that thecurrent density, at-the electrodes be maintained at less than onehalfmilliampere per square centimeter.

The electrode material should preferably be of an oxidizable material,such as aluminum, tantalum, etc. The higher the resistance of the filmover the electrodes, the less becomes the effect of variations incontact resistance, when employing high-frequency, alternating currentas contemplated by this invention.

It is usually advantageous to tune the measuring system (whichincludes-the measuring apparatus, cable and reel, and electrode system)to approximately the resonant frequency. This .allows more accuratemeasurement of the alternating current characteristics which vary withthe capacity changes, caused by variations in the dielectric propertiesof the strata within the sphere of influence of the electrode system.

In obtaining measurements according to any one of the specificprocedures above described, it will be seen that a system of spacedelectrodes is lowered into the drill-hole, and measurements are madewith said electrodes at different depths. The measurements obtained ateach position of the electrodes afford an indication of the alternatingcurrent impedance, dielectric properties, or other alternating currentcharacteristic of the materials comprising an elementary portion of theearth formation penetrated by said drill-hole, said elementary. portionconstituting the portion of the earth formation electrically includedbetween said electrodes at that position. By changing the depth of theelectrodes and makin measurements with the electrodes at differentdepths (preferably in such manner as to obtain a continuous record ofthe measured characteristic as the electrodes are progressively loweredor raised), so as to measure an alternating current characteristic ofdifferent elementary portions of the penetrated earth formation, I amthus enabled to determine variations in the measured characteristic withrespect to depth (preferably as a continuous record of saidcharacteristic at varying depth), throughout any desired length of thedrill-hole and the earth formation penetrated thereby.

I claim: a

1. An alternating-current process for determining the character andthickness of the geological formations traversed by uncased drillholes,which consists in applying alternating current successively to differentelementary portions of the formation adjacent such a drill-hole, andmeasuring the alternating-current phase angles between current andpotential, while maintaining the system at approximately the resonantfrequency of the measuring system, due to the formations encountered atdifferent depths inside the drill-hole.

2. An alternating-current process for determining the character andthickness of the geological formations traversed by uncased drillholes,which consists-in applying alternating current to an electrode systemdisposed within such a drill-hole, moving said electrode system todifferent depths in said drill-hole, and measuring the relativealternating-current power losses caused by the different formations assaid electrode system traverses the drill-hole.

3. An altemating-current process for determining the character andthickness of the geological formations traversed by uncased drillholes,which consists in applying alternating current to an electrode systemdisposed within such a drill-hole, moving said electrode system -todifferent depths in said drill-hole, and measuring the relativealternating-current power losses at approximately the resonant frequencyof the measuring system caused by the different formations, as saidelectrode system traverses the drill-hole.

4. An alternating-current process for determiningthe character andthickness of the geological formations traversed by uncased drillholes,which consists in applying alternating current to an electrode systemdisposed within such a drill-hole, moving said electrode system todifferent depths in said drill-hole, and measuring the relativealtemating-current power losses at a constant frequency caused by thedifferent formations, as said electrode system traverses the drill-hole.

5. An alternating-current device for determining the character andthickness of the geological formations traversed by an uncaseddrillhole, comprising two electrodes of extended area;

means for varying the depth of these two electrodes in the drill-hole;means for measuring the electrical impedance of the materials at andadjacent to the electrodes, whereby an approximate value for thealternating-current impedance of the formations at the depth of the twoelectrodes can be deduced.

6. An alternating-currentdevice for determining the character andthickness of geological formations traversed by an uncased drill-hole,comprising two electrodes of extended area; means for varying the depthof these two electrodes in the drill-hole; means for measuring theelectrical impedance, at two or more frequencies, of

- the materials adjacent to the electrodes, whereby an approximate valuefor the specific dielectric constant of the materials can be deduced.

7. A method for determining variations in earth formations penetrated bya drill-hole, which comprises lowering a system of electrodes in adrillhole, moving said electrodes to different depths within saiddrill-hole, supplying alternating current to said electrodes atdifferent depths, and measuring the alternating current impedance of theelementary portion of the penetrated earth formation includedelectrically between said electrodes at each of said depths.

8. A method for determining variations in earth formations penetrated bya drill-hole, which comprises lowering a system of electrodes in adrill-hole, moving said electrodes to different depths withinsaiddrill-hole, supplying alternating current to said electrodes atdifferent depths, and measuring changes in the alternating current phaseangle caused by the elementary portion of the penetrated earth formationincluded electrically between said electrodes at each of said depths.

9; A method for determining variations in earth formations penetrated bya drill-hole, which comprises lowering a system of electrodes in adrill-hole, moving said electrodes to different depths within saiddrill-hole, supplying altemating current to said electrodes at different"1| depths, and measuring changes in the altemating current power lossescaused by the elementary portion of. the penetrated earth formationincluded electrically between said electrodes at each of said depths.

10. A method for determining variations in earth formations penetratedby a drill-hole, which comprises lowering a system of electrodes in adrill-hole, moving said electrodes to different depths within saiddrill-hole, supplying alterhating current to 'said electrodes atdifferent depths, and measuring changes in the dielectric properties ofthe elementary portion of the penetrated earth formation includedelectrically between said electrodes at each of said depths.

11. A method for determining variations in earth formations penetratedby a drill-hole, which comprises lowering a system of two pairs ofelectrodes in the liquid within a drill-hole, moving said electrodes todifferent depths within said drill hole, supplying high-frequencyalternating current to one pair of electrodes at different depths, andmeasuring the potential created across an elementary portion of thepenetrated earth formation included electrically between the remainingpair of electrodes at each of said depths.

12. The invention as set forth in claim 11, with electrodes,

the added provision that the frequency be held constant atone valueduring descent of the hole, and at another value during ascent of the.hole.

13. A method for determining variations in earth formations penetratedby a drill-hole, which comprises lowering a system of electrodes in theliquid within a drill-hole, moving two or more of said electrodes todifferent depths within said drill-hole, energizing the earth adjacentsaid drill-hole with high-frequency alternating current, and measuringthe potential created across an elementary portion of the penetratedearth formation included electrically. between two electrodes at each ofsaid depths.

14. The invention as set forth in claim 13, with the added provisionthat the frequency be held constant at one value during descent of thehole, and at another value during ascent of the hole.

15. The method of determining the physical characteristics ofsubterranean formations adjacent a bore hole which includes: lowering apair JOHN JAY JAKosKY;

